我已经在模块中导入了mat图标模块，并且我尝试在少数组件内部使用mat-icon，并在其中嵌套文件夹。但它给出了错误。
Tried declaring at root level in app.module.
'mat-icon' is not a known element:
1. If 'mat-icon' is an Angular component, then verify that it is part of this module.
<div class="upload-icon">[ERROR ->] <mat-icon aria-label="arrow upward icon">arrow_upward</mat-icon></div>